Disposable sensor detects “shoe bomb” explosive

CHAMPAIGN, Ill. – A new disposable postage stamp-size colorimetric sensor array can detect
a high explosive in its vapor phase, even at very low concentrations. The scientists
at the University of Illinois at Urbana-Champaign who created it also have designed
a prototype handheld scanner that can read the results.

The explosive, triacetone triperoxide, or TATP, was made infamous
by al-Qaida member Richard Reid, better known as the “shoe bomber.”
On Dec. 22, 2001, while airborne on American Airlines Flight 63 from Paris to Miami,
he tried to detonate the explosives hidden in his shoes.

TATP vapor concentrations are indicated by the patterns and shades
of color on these postage stamp-size disposable sensors. Images courtesy of the
University of Illinois at Urbana-Champaign.
First synthesized in 1895, TATP has been used in at least three
major terrorist acts over the past decade. Not only are its chemical ingredients
readily available, but also it is difficult to detect. It does not fluoresce or
absorb UV light and, unlike other nitro explosives, is not traceable by ion mobility
spectrometry. Although there are commercial methods that can identify TATP, they
cannot detect it in its vapor form and are expensive and nonportable.

The new sensor is a by-product of the researchers’ long-term
goal of creating a personal safety device for personnel who are exposed to volatile
chemicals in the workplace. To that end, they have been developing sensors comprising
arrays of tiny dots, each of which is a unique chemical substance that changes color
when oxidized – similar to litmus paper, which changes color in an acid or
base.

“We turned our attention to TATP because we realized that
there was no current technology capable of rapid and sensitive detection of peroxide
explosives and because we believed that our colorimetric approach could be easily
extended to this analyte,” Suslick said.

Enabling the breakthrough was the discovery that the solid-acid
catalyst Amberlyst-15 converts the relatively unreactive vapor form of TATP to the
much more easily detected hydrogen peroxide, plus acetone – acids that register
readily on a colorimetric sensor.

With the benefit of this advance, the researchers produced a 4
x 4 array of chemically responsive dot-size colorants on a sensor measuring approximately
1 x 1 cm. With an ordinary flatbed scanner, images were made of the dots before
and after exposure to various concentrations of TATP vapor.

Each concentration level, they determined, has its own distinct
pattern. “The pattern of the color change is a unique molecular fingerprint
for TATP at any given concentration, and we can identify it in a matter of seconds,”
Suslick said.

By measuring the red, green and blue values of each dot, they
created color difference maps that enabled them to detect TATP vapor concentration
levels lower than 2 parts per billion. Data was verified with in-line analysis in
real time using a Fourier transform IR multigas analyzer.

As for the handheld scanner developed by the team, it consists
of a CMOS camera – similar to what is found in a cell phone – illuminated
by a white LED. Despite its simplicity, it has a signal-to-noise ratio that is three
times better than that of the flatbed scanner used in the study.

Potential applications of this handheld scanner include airport security and biomedicine.
The handheld scanner is being commercialized by iSense, a company
co-founded by Suslick. The university has granted iSense an exclusive license to
commercialize the technology, which is still in the R&D phase. Although airport
security is one potential application, the company’s main focus is on biomedical
uses, such as rapid identification of bacteria.

The study, titled “A Colorimetric Sensor Array for Detection
of Triacetone Triperoxide Vapor,” was published in the Nov. 10, 2010, issue
of the Journal of the American Chemical Society. Its co-author was postdoctoral
researcher Hengwei Lin.